Miocene

The dragon blood tree is considered a remnant of the Mio-Pliocene Laurasian subtropical forests that are now almost extinct in North Africa. (By Boris Khvostichenko(User:Boriskhv) (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons)

The Miocene Epoch refers to a span of geologic time from 5.3 million to 23.0 million years ago. Figure 1 shows the major geologic epochs of the Cenozoic Era and the relative position of the Miocene.

Figure 1. Start times of the major epochs during the Cenozoic Era. (Image Copyright: Michael Pidwirny).

Stratigraphy

The Miocene was first recognized and defined by Charles Lyell in the early nineteenth century. While examining rocks in the Paris Basin, he noted that different strata contained varying percentages of living mollusc species. The Miocene consisted of layers in which only 18% of the fossils were represented among living mollusc species.

Stratigraphy within the Miocene, as with much of the Cenozoic, is often defined on a highly regional basis. Terrestrial faunas are recognized in ages that vary from continent to continent, primarily because the animals themselves varied from place to place. These ages are usually defined on the basis of the land mammals, so that North America, Europe, Australia, etc. each have their own Land Mammal Ages.

For marine stratigraphy, diatoms and foraminifera are the primary groups used to recognize ages. By this time, both groups were abundant and diversified globally, so much so that diatomite is a common marine sediment of the Miocene. Because the diatoms are abundant, and are the substance of which many marine deposits are formed, they are particularly useful for identifying the relative ages of fossil deposits.

Life in the Miocene

Many of the fossils of species of this time have close relatives alive today, which can be both good and bad. It is good in that we have living representatives to infer past biology, but bad in that we may be tempted to infer too much from this.

The overall pattern of biological change for the Miocene is one of expanding open vegetation systems (such as deserts, tundra, and grasslands) at the expense of diminishing closed vegetation (such as forests). This led to a re-diversification of temperate ecosystems and many morphological changes in animals. Mammals and birds in particular developed new forms, whether as fast-running herbivores, large predatory mammals and birds, or small quick birds and rodents.

Figure 2 shows a Chalicothere, a Miocene mammal from Kazakhstan. Chalicothere was an odd-toed hoofed mammal, or Perissodactyl. Modern day ancestors of this organism include horses, zebras, rhinos and tapirs. Both the Perissodactyls and Artiodactyls (cloven-hooved mammals, modern day representatives include sheep, goats, camels, pigs, cows, deer, giraffes and antelopes) underwent a period of rapid evolution during the Miocene.

Plant studies of the Miocene have focused primarily on spores and pollen. Such studies show that by the end of the Miocene 95% of modern seed plant families existed, and that no such families have gone extinct since the middle of the Miocene. A mid-Miocene warming, followed by a cooling is considered responsible for the retreat of tropical ecosystems, the expansion of northern coniferous forests, and increased seasonality. With this environmental change came the diversification of modern graminoids, especially grasses and sedges.

Because of plate movements, the African-Arabian plate joined with Asia during the Miocene closing the seaway which had previously separated these two landmasses. This new land connection allowed for previously separated faunas and floras to come into contact with each other and resulted in a number of organism migrations.

In addition to changes on land, important new ecosystems in the sea led to new life-forms there. Kelp forests appeared for the first time, as did sea otters and other organisms unique to those environments. At the same time, such ocean-going mammals as the Desmostylia went extinct (Figure 3).

Paleoclimate and Geology

The Miocene was a time of warmer global climates than those in the adjacent Oligocene and Pliocene epochs. During the Miocene two major ecosystems first appear: kelp forests and grasslands. The expansion of grasslands during this epoch is correlated to a drying of continental interiors as the global climate first warmed and then cooled.

The Miocene saw a change in global circulation patterns due to slight position changes of the continents and globally warmer climates. Conditions on each continent changed somewhat because of these positional changes, however it was an overall increase in aridity through mountain building that favored the expansion of grasslands. Because the positions of continents in the Miocene world were similar to where they lie today, it is easiest to describe the plate movements and resulting changes in the paleoclimate by discussing individual continents.

In North America, the Sierra Nevada and Cascade Mountain ranges formed, causing a non-seasonal and drier mid-continent climate. The increasing occurrences of drought and an overall decrease in absolute rainfall promoted drier climates. Additionally, grasslands began to spread, and this led to an evolutionary radiation of open habitat herbivores and carnivores. The first of the major periods of immigration via the Bering land connection between Siberia and Alaska occurred in the middle of the Miocene, and by the end of the Miocene the Panama isthmus had begun to form between Central and South America.

Plate tectonics also contributed to the rise of the Andes Mountains in South America, which led to the formation of a rain shadow effect in the southeastern part of the continent. The movement of the plates also facilitated trends favoring non-desert and highland environments.

In Australia, the climate saw an overall increase aridity as it continued to drift northwards, though the continent went through many wet and dry periods. The number of rainforests began to decrease, to be replace by dry forests and woodlands. The vegetation began to shift from closed broad-leaved forests to more open, drier forests as well as grasslands and deserts.

Eurasia also experienced increasing aridification during the Miocene. Extensive steppe vegetation began to appear, and the grasses became abundant. In southern Asia, grasslands expanded, generating a greater diversity of habitats. However, southern Asia was not the only area to experience an increase in habitat variability. Southern Europe also saw an increase in grasslands, but maintained its moist forests. Although most of Eurasia experienced increasing aridity, some places did not. The climate in some Eurasian regions, such as Syria and Iran, remained wet and cool.

During the Miocene, Eurasia underwent some significant tectonic rearrangements. The Tethys Sea connection between the Mediterranean and Indian Ocean was severed in the Mid-Miocene causing the increase in aridity in Southern Europe. The Paratethys barrier, which isolated Western Europe from the exchange of flora and fauna, was periodically disrupted, allowing for the migration of animals. Additionally, faunal routes with Africa were well established and occasional land bridges were also created.

Africa also encountered some tectonic movement, including rifting in East Africa and the union of the African-Arabian plate with Eurasia. Associated with this rifting, a major uplift in East Africa created a rain shadow effect between the wet Central-West Africa and dry East Africa. The union of the continents of Africa and Eurasia caused interruption and contraction of the Tethys Sea, thereby depleting the primary source of atmospheric moisture in that area. Thus rainfall was significantly reduced, as were the moderating effects of sea temperature on the neighboring land climates. However, this union enabled more vigorous exchanges of flora and fauna between Africa and Eurasia.

Antarctica became isolated from the other continents in the Miocene, leading to the formation of a circumpolar ocean circulation. Global ocean and atmospheric circulation were also affected by the formation of this circumpolar circulation pattern, as it restricted north-south circulation flows. This reduced the mixing of warm, tropical ocean water and cold, polar water causing additional buildup of snow and ice on Antarctica. This accelerated the development of global seasonality, aridity, and enhanced global cooling.

This entry is based on an article written for the University of California Museum of Paleontology by Brian R. Speer, Lucy Brining, Valerie Chan, Ellen Choi, Michael De Sosa, and Christina Lee.